Vehicle occupant restraint systems having an actuatable restraining device are well known in the art. One particular type of actuatable restraining device includes an inflatable air bag mounted within the occupant compartment of the vehicle. The air bag typically has an associated, electrically actuatable igniter, referred to as a squib.
Such occupant restraint systems further include an inertia sensing device for measuring deceleration of the vehicle and providing an electrical signal indicative of such deceleration. When the inertia sensing device indicates that the vehicle's deceleration is greater than a predetermined value, an electric current of sufficient magnitude and duration is passed through the squib for the purpose of igniting the squib. The squib, when ignited, ignites a combustible gas generating composition and/or pierces a container of pressurized gas operatively coupled to the air bag, which results in inflation of the air bag.
It is not desirable to inflate a vehicle air bag under all types of vehicle crash conditions. A vehicle crash condition in which it is not desirable to inflate the air bag is referred to as a non-deployment crash condition. A vehicle crash condition in which it is desirable to deploy the vehicle air bag is referred to as a deployment crash condition. A determination as to which vehicle crash conditions are non-deployment crash conditions and which are deployment crash conditions is dependent upon various factors related to the type of vehicle. If, for example, a large vehicle traveling eight miles per hour hits a parked vehicle, such a crash condition would typically be considered a non-deployment crash condition. The vehicle seat belts alone would be sufficient to provide occupant safety in such a crash condition.
It is important, therefore, to be able to discriminate between types of vehicle crash conditions so as to control (i) whether or not the occupant restraining device is actuated and (ii) the timing of the actuation of the occupant restraining device. Timing of the actuation of the occupant restraining device is particularly important in an air bag system.
The inertia sensing device in an actuatable occupant restraint system is a basic means for discriminating a vehicle crash condition. Many known inertia sensing devices used in occupant restraint systems are mechanical in nature. Such devices are typically mounted to the vehicle and include a pair of switch contacts and a resiliently biased weight. The weight is arranged such that when the vehicle decelerates, the weight physically moves relative to its mounting. The greater the deceleration, the farther the weight moves against the bias force. The switch contacts are mounted relative to the biased weight such that, when the weight moves a predetermined distance (i.e., when there is a deceleration greater than a predetermined amount), the weight moves over or against the switch contacts, causing them to electrically close. The switch contacts, when electrically closed, connect a squib to a source of electrical energy sufficient to ignite the squib. Therefore, this type of inertia sensor discriminates a vehicle crash condition by closing switch contacts when vehicle deceleration is greater than a predetermined amount, i.e., the amount necessary to move the weight to the point of electrically closing the switch contacts.
Other known occupant restraint systems for vehicles include an electrical transducer or accelerometer for sensing vehicle deceleration. In such systems, vehicle deceleration is used for discriminating a vehicle crash condition. Such systems include a monitoring or evaluation circuit connected to the output of the transducer. The transducer provides an electrical output signal having a value indicative of the vehicle's deceleration. The monitoring circuit processes the transducer output signal. One typical processing technique is to integrate the transducer output signal using an analog integrator. If the output of the integrator exceeds a predetermined value, thereby indicating that the vehicle deceleration is greater than a predetermined amount, an electrical switch is actuated so as to connect electrical energy to a squib.
U.S. Pat. No. 5,036,467 to Blackburn et al. discloses a method and apparatus for discriminating vehicle crash conditions in real time using a frequency domain integration and summation algorithm. An accelerometer provides a vibratory time domain electric signal having frequency components indicative of a vehicle crash condition. An A/D converter converts the accelerometer signal into a digitized signal. A fast Fourier transform device transforms the digitized time domain vibratory electric signal over at least two time intervals into frequency domain signals. The amplitudes of all frequency bins over the entire frequency spectrum for each frequency domain signal are summed to provide a value corresponding to the integral of the amplitudes. The values of the integrals of the frequency domain signals are summed. A microcomputer monitors the sum of the integral values of the frequency domain signals. The microcomputer actuates the occupant restraint system when the sum of the integral values of the frequency domain signals is greater than a predetermined threshold, thereby indicating a particular type of vehicle crash is occurring.